The present invention relates generally to printed circuit boards and circuit assemblies having imbedded components. Imbedded components are herein defined as active and passive electrical devices, both in packaged and unpackaged states, wherein the devices are placed within cavities of a laminated circuit board. Examples of components that may be imbedded are: bare die, chips, thick or thin film devices and all constructed devices, including crystals, oscillators and wire wound devices. More particularly, this invention pertains to coated and encapsulated printed circuit boards having imbedded components. Even more particularly, this invention pertains to coated and encapsulated printed circuit boards having imbedded components and imbedded thermal cores.
The emergence of the application of multi-layer laminates in printed circuit board design has placed much more emphasis on printed circuit board reliability. Printed circuit board operational stress factors affecting reliability include internal operating conditions including overall and component level power generation and heat dissipation and environmental factors like thermal cycle, mechanical shock, vibration, humidity and contaminants. This emphasis has been compounded by the direction of the industry to fabricate smaller, denser printed circuit board packages, especially for use in mobile platforms such as cell phones, vehicles and airframes. For example, use of printed circuit board in avionics requires such boards be designed and constructed so as to minimize volume and mass, to operate in conditions of significant vibration and high G-forces, and to withstand conditions of widely varying pressure, temperature and humidity. Incorporation of high speed processors requires compatible integrated circuit speeds and signal densities.
Conventional printed circuit board designs can not optimally meet these design goals. Conventional printed circuit board design frequently employs surface mount and through-hole mount technologies to physically secure and electrically connect circuit components to the integrated circuit imbedded in the laminate substrate. Surface mounting methods and through-hole mounting methods include the use of substantial packaging interconnects having one or more series of electrical interconnects. Typical of this technology are semiconductor dies attached to a leadframe and encased in molded plastic body, wherein the leadframe is soldered to the printed circuit board. The leadframe and the molded plastic body add to circuit board weight, height and volume. The leadframe also adds to overall circuit resistance and self impedance. The large size of leadframe connections limits the I/O density of the circuit.
More recently, die packaging technology has incorporated technologies such as flip chip design, wherein solder spheres directly connect a semiconductor die to a circuit bonding pad. Ball grid arrays have reduced the interconnect length and the number of packaging interconnects. However, there have been unanticipated limitations with this technology when exposed to environmental stresses. The solder spheres tend to concentrate shock, vibrational stress and thermal stress and frequently fail during or following exposure to such operating conditions. These limitations are particularly prevalent in printed circuit boards used in portable systems such as cell phones, radios, and avionics.
What is needed, then, is a printed circuit board assembly which has: reduced mass and volume as compared to conventional standard circuit boards; higher circuit I/O densities; improved shock loading, vibrational and thermal stress performance, improved thermal management, and multiple levels of environmental protection and electrical insulation.